The present invention relates to a system for metal powder atomisation comprising a melting furnace with a drain arranged for draining molten metal from the bottom of the furnace. The invention also relates to a method of metal powder atomisation using such a system.
The properties of many steels and other metal alloys are easily degraded by an excess amount of oxygen and sometimes nitrogen in the alloys. For example, the bend strength and impact strength of high speed steels risk to be reduced by oxygen-rich inclusions and stainless steels have their corrosion resistance and mechanical properties degraded by high oxygen content. Certain alloys containing easily oxidisable elements such as Ti, Al, or rare earth elements are not possible to atomise at a reasonable cost in large scale using conventional powder atomisation processes due to low yield and bad predictability of the oxidising elements.
There are many different ways of producing metal powder, such as jet casting, centrifugal casting, water atomisation, oil atomisation, ultrasonic atomisation, and gas atomisation. Gas atomisation is known to produce a spherical powder with relatively low oxygen levels in comparison with powder atomised using for example water atomisation. For large scale atomisation with large batch sizes, typically several tonnes, metal is molten under air in an induction furnace. The thus obtained liquid metal is poured over spout by tilting the furnace. The metal is poured either directly into a tundish or via a ladle. The liquid metal in the tundish is then drained through an opening in the bottom of the tundish into the upper part of an atomisation chamber. Upon entry into the atomisation chamber, the liquid metal is hit by a gas at high velocity, whereby a spherical powder is produced.
During the pouring over spout from the furnace to the tundish or ladle, the entire batch of liquid metal is exposed to the surrounding atmosphere. If the surrounding atmosphere is air, the oxygen content of the liquid metal increases. In JP7048610, this problem is handled by placing a tiltable furnace and a tundish together in an enveloping chamber, in which a protective atmosphere is maintained. Thus, the liquid metal is not subjected to oxygen upon pouring over spout from the furnace to the tundish. However, this system is not adapted to use for large scale atomisation (>500 kg), since the enveloping device in this case becomes very large. It is also difficult to sample and adjust the alloy in the melting furnace inside the chamber.
Another way of handling the problem is to use a melting furnace from which the liquid metal is drained through a drain in the bottom of the furnace. The exposure of the entire batch of liquid metal to the atmosphere is thereby avoided. In order to keep the liquid metal in the furnace during melting, sampling and adjustment of the composition, a stopper rod is introduced into the drain. The stopper rod is a vertical rod operable from the top of the furnace. The stopper rod can be removed by pulling it upwards, thus opening the drain. However, it is difficult to charge the furnace without damaging or breaking the stopper rod. It is also difficult to make large scale furnaces reliably functional, since the stopper rod must be upsized with the furnace. Since the rod has to withstand high temperatures, it must be made of a refractory material. Ceramic refractory materials are typically associated with problems such as brittleness and metallic refractory materials are known to oxidise and to be very expensive.
U.S. Pat. No. 4,562,943 discloses a melting furnace with an opening in the bottom through which the melt can be poured. The opening is blocked by a closure plate, which is configured to melt as a result of heat transferred from the melt. To prevent the closure plate from melting, it is cooled from below until pouring is to be started. To start pouring, the cooling is decreased or turned off and as a result, the closure plate is melted by heat transferred from the melt and pouring is initiated. However, the high temperature gradients present in such a furnace make it very difficult to control. A too thick closure plate will be impossible to melt through by heat transfer from the melt in order to start bottom pouring. On the other hand, a too thin plate or too little cooling may start an uncontrolled bottom pouring.